The use of optical signals for communications and for other purposes is increasing. Typically, optical signals are transmitted from one location to another via fiber optic devices, an example of which is an optical fiber. Optical circuits and other optical equipment used in optical communication systems include, for example, multiplexers, optical switches, cross connect devices, and/or communications to, form and/or between (or among) computers, and the like. Sometimes it is necessary to test the fiber optic devices themselves and/or the other optical equipment by measuring light. An exemplary device currently used to test fiber optic devices and components is the Hewlett-Packard model 81641A lightwave measurement system. Such measurement system and other optical measurement systems use a tunable laser system to supply light in the visible or other wavelength range to a device under test (sometimes referred to below as “DUT”), such as an optical fiber and/or other optical equipment. The output from the DUT may be measured using one or more optical power meters (sometimes referred to below as “OPM”). Fiber optic devices may be used to connect the tunable laser system to the DUT and to connect the DUT to one or more OPMs. It is desirable that the measurements made by the OPMs, for example, which measure optical power or intensity of an input, be reproducible, sometimes referred to as repeatable, for a given set of test conditions, such as, for example, wavelength and intensity or power of light from the tunable laser system and positioning, orientation, operation and/or setting of the DUT. However, it has been found that some errors have occurred in the past that negatively affected the reproducibility and, thus, the reliability of the measurements.
In FIG. 1 a prior art optical measurement system 10 is shown including a light input represented by an arrow 11, an optical integrating sphere 12, a light output 13, and an OPM 14, which includes a photosensitive detector 15, measurement circuitry 16, and an output circuit and display 17. The light input, such as a connection to a fiber optic device, is provided the conventional optical integrating sphere 12 at its light input port or opening 18, which is located to receive the light directed generally along an axis that is the same as a diameter of the integrating sphere. The light output 13 opening in the integrating sphere is along another diameter, which is perpendicular to the first-mentioned diameter. Input light reflects on the walls 19 facing the interior 20 of the integrating sphere 12 and eventually at least some of the light is directed out through the light output 13 for detection by the photosensitive device 15, which is outside the integrating sphere 12, as is represented by the dotted lines in FIG. 1.
In this patent application reference is made to electromagnetic energy, light, optical, optical power meter, and the like. It is intended that such references identify, broadly, electromagnetic energy that may be in the visible spectrum, in the infrared spectrum, including without limitation the near infrared and the far infrared spectra, ultraviolet spectrum, and/or the like. Thus, reference to light, optical, and/or to electromagnetic energy is not restricted to light in the visible range but also to light or electromagnetic energy in other ranges of the electromagnetic energy spectrum.
Typically an optical fiber either has an integrated connector or is put into a bare fiber adapter, which holds the fiber in a fixed location. Such integrated connector or bare fiber adapter is used to connect the optical fiber to a DUT and/or to an OPM in a test system of the type mentioned above. Measurement errors that affect reproducibility of measurements sometimes arise on account of different alignments and relationships of parts at connections between the fiber optic device and the DUT or OPM. One source of error is the different alignments of the end of the fiber optic device at the connection to the OPM or to the DUT, which is manifest by wiggling the connector or the fiber optic cable at the connection and observing changes in a measurement.
As is illustrated schematically in FIGS. 2A, 2B and 2C, sources of error in prior art optical measurement systems include Z-axis position of the optical fiber 21 output end portion or end face 22 (sometimes referred to as “output face” or “output end”) of the optical fiber 21, X-Y position of the end face, bending of the end portion of the end portion, and/or rotation of the optical fiber about the Z-axis. Sometimes the end face 22 is cut on a bias or slope rather than being perpendicular to the axial extent of the optical fiber, and such slope further exacerbates the error problems. The end face 22 of the optical fiber 21 usually has been located in the input port 18 of an integrating sphere 12 associated with an OPM 14; and light from the end face 22 usually is distributed over a cone shape represented at 23 in FIG. 2A. Depending on the location and/or orientation of the end face 22 in the input port 18, some light from the cone 23 may impinge on and reflect from walls 24 of the input port; for different locations and orientations of the end face, then, it will be appreciated that the extent of such impingement and reflection will be different and may result indifferent measurements by the OPM 14 for the same intensity or power of input light provided by the optical fiber.
In FIG. 2A the Z-axis location of the end face 22 of the optical fiber 21 is shown at two different locations i and ii relative to the axial extent of the input port 18. With the end face 22 at the Z-axis location i, part 23i of the cone 23 of light impinges on and reflects from walls 24 of the light input port 18, and the amount of light that so impinges and reflects depends on the Z-axis location of the end face 20. In contrast, with the end face 22 at Z-axis location ii, none of the cone 23 of light, as is represented at 23ii, does impinges on the walls 19, but rather the entire cone 23 of light directly enters into the interior 20 (FIG. 1) of the integrating sphere 12. Another similar source of error is the lateral offset of the end face 22 in the X- or Y-axial directions relative to the Z-axial extent of the light input port 18. FIG. 2B illustrates a bending of the optical fiber 21 relative to the linear axial extent of the light input port 18. In FIG. 2C the end face 22 of the optical fiber 21 is cut (faceted) on an angle (bias) other than perpendicular to the Z-axis direction so that light input to the interior 20 of the integrating sphere 12 will be directed away from that portion 19a of the wall 19 (FIG. 1), which is directly opposite the light input port 18, to avoid direct reflection back into the input port and subsequent loss of some light. However, as is represented by the arrow 27, polar rotation of the optical fiber 21 about the Z-axis may cause a corresponding variation in the angle at which the end face 22 faces to change the path that light takes into the integrating sphere 12 and, thus, may cause variation in the amount and direction of light that impinges on the walls 24.
Each of the above-mentioned variations depicted in FIGS. 2A, 2B, and 2C ordinarily would have minimal impact, if any impact at all, on measurements made by the photosensitive detector 15 of the OPM 14, provided the size of the integrating sphere 12 is relatively large, for example, larger than about 1½ inches or 2 inches interior diameter or even larger. However, as a conventional integrating sphere is miniaturized, for example, to a size on the order of about 1½ inches or smaller, even 1 inch or smaller, then such variations depicted in FIG. 2A, 2B, and 2C may have a substantial impact and, therefore, reduce reproducibility of measurements based on a light input with common characteristics.
Light integrating spheres have been used in the past to make high precision optical measurements. The integrating spheres have been relatively large, for example, larger in diameter than about 1.5 inches or 2 inches. The relatively large size of the integrating sphere in such a measurement instrument has been necessary to obtain substantial light diffusion, e.g., on account of number of bounces of light beams in the integrating sphere, thus minimizing the light lost by reflection back out through the light input port, avoiding light direction or light input device alignment affects, reducing the impact of optical polarization, and so forth. However, large integrating spheres are relatively expensive. Also, a measurement instrument that uses a large integrating sphere usually only has space for a single integrating sphere and, therefore, only one measurement can be made at a time, which slows the operation and limits versatility of the measurement instrument. Accordingly, it would be desirable to be able to use smaller integrating spheres in optical measurement equipment so that several measurements can be made simultaneously or nearly simultaneously, thus increasing the effective speed of the instrument operation and increase versatility of the instrument and its measurements. Even for a single measuring channel optical measurement instrument, it would be desirable to be able to use a smaller integrating sphere than was previously possible thereby to reduce instrument size.
However, miniaturization of the integrating sphere used in such measurement instruments may reduce the light diffusing and other intended functions of the integrating sphere, and, therefore, may reduce the reproducibility of measurements made by OPMs associated with reduced size integrating spheres. Several problems encountered due to miniaturization of integrating spheres for optical measurement instruments include, for example, increased percentage of reflected light lost through the light input port, polarization dependency, light input device position and alignment dependency, e.g., as shown in FIGS. 2A, 2B and/or 2C, and light loss at the detector port. Thus, there is a need to reduce these losses and errors to improve the reproducibility, repeatability and reliability of measurements made by optical measurement instruments, which utilize miniature integrating spheres.
The input light provided an integrating sphere and OPM may be optically polarized. The light reflection by the integrating sphere may depend on the polarization direction. Also, the sensitivity of the light detector in the OPM may be polarization direction dependent, and reflection from the detector itself may be polarization direction dependent. It is desirable to increase the number of reflections in the integrating sphere to randomize or to scramble the polarization direction and, therefore, to reduce the polarization dependency on light measurements by the OPM. Although relatively large integrating spheres tend to provide enough reflections to randomize the optical polarization, the smaller the integrating sphere, in the past the less randomizing would occur. Accordingly, it is desirable to provide for such randomizing for relatively small integrating spheres.
Light loss may occur at the light input port of an integrating sphere; some of the light is reflected back to the input port. Light leakage out the inlet port, such as the mentioned light which is reflected back; results in error in optical measurement. The smaller the integrating sphere, usually the larger the input port is relative to the total surface area of the integrating sphere and also the closer the input port is to the immediately opposite or back surface of the integrating sphere; and both of these conditions increase the quantity of light that may be directed back to the input port.
The light exiting an optical fiber usually forms a cone of light. If the size of the inlet port to an integrating sphere is relatively small to avoid reflected loss back out through the input port, then the above-mentioned possible misalignment of the output end of the optical fiber may result in light being reflected by surfaces of the light inlet port and the above-mentioned measurement errors.
Another loss encountered in optical measurement instruments, which use integrating spheres, is the loss around the detector port or light output port. Such loss may relate to the fact that there may be less than a perfect fit between the output port size and the light detector, thus allowing some light leakage. This light leakage as a percentage of the total area of the integrating sphere increases with miniaturization of the sphere. It is desirable to reduce such leakage and associated errors.